A greater understanding of equatorial teleconnections is a key aspect of research into seasonal prediction and future climate change for tropical regions. Here the impact of Pacific sea surface temperature (SST) anomalies on rainfall fluctuations over the semi-arid Sahel is explored, using a combination of observed and model data.
The first key topic of this study is the identification of those aspects of anomalous Pacific SST variability that are most strongly linked to the Sahel. One of these, also illuminated by earlier studies, is similar to the classic El Niño Southern Oscillation pattern in the central and east Pacific, which in its El Niño phase increases the likelihood of Sahel drought. It is shown here that, although a part of this link is indirect (operating via Atlantic SSTs), its main effect appears to be through a direct atmospheric teleconnection. The other critical pattern, of equal importance and revealed here by a novel analysis technique, is the large-scale zonal gradient of SSTs from the west Pacific to the east Indian Ocean. If weakened, this too enhances the likelihood of Sahel drought. Atmospheric general circulation model experiments, forced either by observed or idealized SSTs, are used to confirm these two influences on the Sahel. Crucially, their Sahelian impact is substantially reinforced when both are present and, additionally, further empirical analysis shows them to be largely independent.
The second key topic is an investigation of the mechanisms for this Pacific-Sahel teleconnection. These appear to involve anomalous stationary equatorial waves, with communication occurring in both the eastward and westward directions. In El Niño years (for example), a Kelvin wave emanates across the Atlantic from east Pacific convective heating anomalies, and an equatorial Rossby wave appears over the Indian Ocean in response to the anomalous west Pacific-Indian Ocean SST gradients via convective heating anomalies over the Indian Ocean. These interact over Africa to enhance large-scale subsidence over the Sahel, thus reducing seasonal rainfall totals. Interannual changes in propagating equatorial waves or in the residence of subseasonal regimes appear not to play a substantial role.